Absorbing shocks and vibrations is a typical problem encountered in mechanics related domains. Most usual solutions are based on combining a spring, for example a helical or a disc-shaped belleville spring, with a unidirectional damper, for example a viscous liquid damper or a rubbery/elastomeric damper. A major disadvantage of these isolators is that they involve complex constructions, especially to ensure liquid or air sealing. In addition, they often involve hitting between elements. Moreover, their properties may depend on the ambient temperature.
Wire rope isolators (WRI's) constitute another common type of mechanical isolators against shocks and vibrations, including for example polycal WRI's, helical WRI's, ring-type WRI's, straight cable WRI's and other special WRI's. The FIG. 1 illustrates a helical WRI of the prior art, along with its main loading directions. The exemplary helical WRI comprises two retainer bar assemblies arranged parallel with an x-axis, each retainer bar assembly comprising holes. The two retainer bar assemblies are bound one to the other by a single cable, the cable bent between the bar assemblies, passing through their holes and generally clamped by each of the retainer bar assemblies using screws. The direction along the x-axis is called the shear direction, the direction along the y-axis is called the roll direction and the direction along the z-axis is called the tension-compression direction. For polycal WRI's, the difference between the roll and the shear directions is less obvious. For ring-type WRI's, the roll and the shear directions are equivalent and best known as the radial direction.
A major disadvantage of WRI's is that they are omnidirectional dampers with directionally dependent stiffness and damping properties, which results in the circumstance that the tuning of an application based on WRI's is difficult.
Yet another disadvantage of WRI's is that the maximum attainable distance from a single interface plane with a given size to the elastic centre of an optimised set-up of spring-damper elements will be smaller for a set-up with omnidirectional springs as compared to an optimised set-up with unidirectional springs. That is, if only a single interface plane is available, then achieving balance of an isolated object requires more space using a set-up with omnidirectional springs. This drawback of the WRI's will be further explicated in the following, as well as how it may be overcome by the present invention.
Yet another disadvantage of WRI's is that the size of an omnidirectional WRI in the tension-compression direction generally becomes significantly larger due to repeated force-deflection cycling in the tension-compression direction and even due to repeated force-deflection cycling in the roll direction. This effect is believed to be caused by plastic deformation of the wires of the steel cable. Because the external load levels and the accompanying material stress levels are much higher upon tension than compression, the plastic deformation tends to increase the size of the WRI in the tension direction. A consequence of this phenomenon is that the average gravity loaded position of an object isolated with omnidirectional WRI's, relative to its direct surroundings, is not constant, but changing with repeated loading during the lifetime of the WRI's. In addition, depending on the location and orientation of the WRI's, the average orientation of the isolated object may be affected. Moreover, due to the increase of size in the tension direction, the amount of travel available for shock isolation in the tension direction becomes smaller than initial, resulting in higher maximum residual accelerations.
In an attempt to overcome some of the aforementioned drawbacks, the U.S. Pat. No. 5,482,259 discloses a unidirectional damper to be used as a pipe restraint, which makes use of the shear direction of a single helical WRI. A major disadvantage of a unidirectional damper according to the U.S. Pat. No. 5,482,259 is that it is hardly applicable to practical shock damping, as shock damping requires a rather high (initial) stiffness, in order to limit the displacements due to gravity and dynamic excitations with low frequency content. In fact, the mass of a unidirectional damper according to U.S. Pat. No. 5,482,259, which would be required to achieve a stiffness value suitable for practical shock damping, would be very large.